The present invention relates to the plaques made of a hard material which are installed in mixer valves for controlling the mixing proportions and the quantity to be delivered, and more particularly to a pair of plaques of hard material for single lever mixer valves of the type in which a first plaque is intended to be installed stationary in the body of the valve and the second plaque is intended to be movably superposed on and in contact with the first plaque and to undergo an angular displacement about a fixed centre of the first plaque, for the adjustment of the mixing ratio, and a linear displacement along a diameter of the second plaque, movable with the latter and passing through the said fixed centre of the first plaque, for the adjustment of the quantity to be delivered, and in which each plaque has two arcuate passage windows, each window of each plaque is intended to cooperate with only one window of the other plaque, and the two windows of each plaque are located at opposite sides with respect to the centre of the respective plaque.
The adjustment fidelity of the valve, i.e. its aptitude not to modify the temperature of the water being delivered, i.e. the mixing ratio, as a consequence of the variation of the adjustment of the flow, and vice versa, depends substantially on the configuration of the passage windows of the plaques. Hitherto attempts have been made to improve the behaviour of the valves of the type specified hereinabove, by correcting in an empirical manner the contour and arrangement of these windows, but no really satisfactory results have been achieved.